AQA Chemistry 5.1.2.5

Group 1: the alkali metals

One outer electron, a little further out each step down. Step down the group, predict each reaction with water, then predict rubidium.

Triple Science · Foundation tier

Start here

A jar of oil.

In a school prep room, small grey lumps of sodium sit at the bottom of a jar, covered in oil.

If you had to guess, is the oil there to stop the metal reacting with air and water, or to keep it shiny for show?

Step down the group

One outer electron. Each step down, it is further out.

The alkali metals all have one electron in their outer shell. They are soft enough to cut with a knife, have low melting points, and lithium, sodium and potassium all float on water. They react with water to make a metal hydroxide and hydrogen. Before each metal goes in, predict.

Lithium first. Ready?

The solution left behind is alkaline: universal indicator turns purple. That is where the name comes from.

Oxygen and chlorine

Not only water.

A freshly cut surface is shiny, then quickly goes dull as an oxide forms. Heated in chlorine, the metals react vigorously to form white solid chlorides. Both reactions get more vigorous down the group. Tap a reaction.

Why reactivity increases

Further away, weaker pull, easier to lose.

Going down the group, the atoms have more shells, so the outer electron is further from the nucleus. The attraction between the nucleus and the outer electron is weaker, so the electron is lost more easily, forming a +1 ion. That makes the metal more reactive.

Predict from a trend

Melting points: lithium 181 °C, sodium 98 °C, potassium 63 °C. Rubidium is next. Predict its melting point.

Think again · more protons, stronger pull?

“Potassium has more protons, so it holds its outer electron more tightly.”

Your call

Choose

Compared with sodium, how strongly is potassium’s outer electron attracted to the nucleus?

Sort

Water, oxygen or chlorine?

Place each product or observation with the reaction it comes from.

Tap a card, then tap the box it belongs in. Tap a placed card to take it back.

Command words in this lesson

Describe: What you would see: fizzing, moving, melting, flame.
Explain: Distance, attraction, how easily the electron is lost.
Predict: Carry the trend on to rubidium and caesium.

Key fact

Group 1 metals have one outer electron. Reactivity increases down the group, because the outer electron is further from the nucleus and is lost more easily.

Key note · AQA 5.1.2.5

Group 1: the alkali metals

Group 1, the alkali metals, have one electron in their outer shell and form +1 ions.
Soft, low density and low melting points. Li, Na and K float on water.
With water: metal hydroxide + hydrogen; the solution is alkaline.
With oxygen: a metal oxide (tarnishes). With chlorine: a white solid metal chloride.
Reactivity increases down the group: the outer electron is further from the nucleus, less strongly attracted, and more easily lost.
Use trends to predict rubidium and caesium.

Photograph this card. Tomorrow, cover it and say each line first.

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Ask MrBadmus

Not sure why potassium is more reactive than lithium?

Shell diagrams are not to scale. The reaction drawings are schematic.